This invention relates to an improved process for the production of maleic anhydride by the oxidation of a hydrocarbon having at least four carbon atoms in a straight chain in a catalytic reactor. More particularly, this invention relates to a method for improving the uniformity of distribution of a phosphorus-containing agent throughout a maleic anhydride catalytic reactor.
Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis.
Maleic anhydride is conventionally manufactured by passing a gas comprising a hydrocarbon having at least four carbon atoms in a straight chain and oxygen through a catalyst bed, typically a fixed catalyst bed tubular plug flow reactor, containing a catalyst comprising mixed oxides of vanadium and phosphorus. The catalyst employed may further comprise promoters, activators or modifiers such as iron, lithium, zinc, molybdenum, chromium, uranium, tungsten, and other metals, boron and/or silicon. The reaction product gas produced contains maleic anhydride together with oxidation by-products such as carbon monoxide, carbon dioxide, water vapor, acrylic and acetic acids and other by-products, along with inert gases present in air when air is used as the source of molecular oxygen.
Because the reaction is highly exothermic, the reactor must be cooled during operation. Typically, a shell and tube heat exchanger is used as a reactor with the catalyst packed in the tubes through which the hydrocarbon and oxygen gases are passed. A cooling fluid, often a molten salt, flows over and cools the outside of the tubes. Because the length of the tubes is generally much greater than the diameter of the tubes, the reaction system approaches plug flow.
While the cooling capacity is substantially uniform throughout the reactor, the rate of reaction varies widely with the concentration of the hydrocarbon reactant and the temperature of the reaction zone. Because the reactant gases are generally at a relatively low temperature when they are introduced into the catalyst bed, the reaction rate is low in the region immediately adjacent the inlet of the reactor. Once the reaction begins, however, it proceeds rapidly with the rate of reaction further increasing as the reaction zone temperature increases from the heat released by the reaction. The reaction zone temperature continues to increase with distance along the length of the reactor tube until the depletion of the hydrocarbon causes the rate of reaction to decrease thereby decreasing the temperature of the reaction zone through transfer of heat to the cooling fluid, and allowing the remaining portion of the reactor tube to operate at a lower temperature differential. The point of maximum temperature reached in the reactor tube is generally referred to as the "hot spot".
If the temperature at the hot spot of the reactor becomes too great, problems can occur in the operation of the reactor. Generally, the selectivity of the catalyst varies inversely with the reaction temperature while the rate of reaction varies directly with the reaction temperature. Higher reaction zone temperatures result in lower catalyst selectivity and favor the complete oxidation of the hydrocarbon feedstock to carbon dioxide and water instead of maleic anhydride. As the hot spot temperature increases, the amount of the hydrocarbon feedstock consumed by the reaction increases but the decreased selectivity of the catalyst can result in a decreased yield of maleic anhydride. In addition, exposure of the catalyst bed to excessive temperatures may degrade the catalyst. Such degradation of the catalyst generally reduces the productivity of the operation and may also reduce the selectivity of the catalyst at a given temperature. Further, because the reaction rate constant increases exponentially with temperature, the reactor can experience thermal runaway if the temperature of the reactor becomes too high. The higher heat of reaction released by the conversion of the hydrocarbon feedstock to carbon dioxide and water further compounds this problem.
To modulate catalyst activity and enhance catalyst selectivity, a small amount of a phosphorus compound can be added to the reactant gases introduced to the reactor. Although the function of the phosphorus compound is not fully understood, it has been hypothesized that phosphorus is lost by the catalyst under the catalytic oxidation conditions and that a portion of the phosphorus compound added to the reactant gases is sorbed by the catalyst. It has been further hypothesized that this treatment of the catalyst with the phosphorus compound increases or restores the phosphorus/vanadium ratio of the catalyst to a ratio more favorable for catalyst selectivity, particularly a ratio that favors formation of maleic anhydride in preference to other catalytic by-products.
This treatment of the catalyst may be further modified by adding both water and a small amount of a phosphorus compound to the reactant gases introduced to the reactor. Although the function of this combination is not fully understood, it has been hypothesized that the addition of water to the reactor gases promotes a relatively even distribution of the sorbed phosphorus compound throughout the catalyst bed. In the absence of moisture, it has been observed that the phosphorus compound introduced into the reactant gases tends to deposit in an area immediately adjacent to the inlet of the tubular reactor.
Edwards, U.S. Pat. No. 4,701,433 and U.S. Pat. No. 4,810,803 disclose the introduction of water and a phosphorus compound into the catalyst bed of a maleic anhydride reactor to partially deactivate a portion of the catalyst bed and to make the temperature profile of the reactor more isothermal. Suitable phosphorus compounds are described to include alkyl phosphites and alkyl phosphates, including trimethyl phosphate. While Edwards discloses that the phosphorus compound and water can be added to the feedstock introduced to the reactor, he further discloses that a variety of other methods can be employed to add the phosphorus compound and water to the catalyst bed. These methods include the use of an aerosol to convey the phosphorus compound; the use of suspensions or colloidal solutions of the phosphorus compound; the use of a solvent for the phosphorus compound; and the addition of the phosphorus compound through a diluent gas such as nitrogen. Edwards does not, however, specifically teach how the phosphorus compound should be added to the feedstock or at what point in the process the phosphorus compound should be added to the feedstock.
Ebner, U.S. Pat. No. 5,185,455 discloses a process for controlling the rate of addition of trimethyl phosphate to an n-butane and oxygen stream entering a maleic anhydride catalytic reactor to improve catalyst selectivity without decreasing catalyst activity. Ebner describes a system for maintaining an optimal concentration of trimethyl phosphate in the reactant gases entering the reactor. Ebner, however, does not specifically teach how the trimethyl phosphate should be added to the feedstock or at what point in the process the trimethyl phosphate should be added to the feedstock.